Fix getSphereOffset rare crash

There was a bug that would rarely occur if a Curve Clear was triggered at the very start of the path.
This was triggerable via fireballs, albeit rarely.

(dear god I hope this doesn't introduce another bug)

Fixes #12

src/SphereGroup.lua

@@ -1083,7 +1083,7 @@ end
 
 
 function SphereGroup:getSphereOffset(sphereID)
-	return self.offset + self.spheres[sphereID].offset
+	return self.offset + ((_MathAreKeysInTable(self.spheres, sphereID) and self.spheres[sphereID].offset) or 0)
 end